When it comes to qualify, hemodialysis and/or hemofiltration membranes have to meet the highest purity requirements. Thus, the alpha-cellulose content must be at least 98%, the degree of polymerization 500 or more, and the contents still remaining in the cellulose may not have any toxic or detrimental medical effects on a dialysis patient.
In the hemodialysis membranes made of regenerated cellulose currently available on the market, the aqueous extracts in an HPLC analysis show a peak for relatively high-molecular-weight substances ranging from 20,000 to 90,000 daltons (Ward, Feldhoff, and Klein: "Role of Dialyzer Contaminants in the Allergic Epiphenomena of Hemodialysis"; Artif. Org. 8, 338 (1984)).
It has also been shown that these extracts show very high positive values in tests with limulus amebocytes lysates (LAL) made by certain manufacturers, such as Travenol or Concept (Heidelberg). The test with Mallinckrodt's LAL shows a negative reaction. Although these tests are normally used for detecting endotoxins and thereby pyrogens, it was demonstrated in pyrogen tests on rabbits that this is a falsely positive reaction. Further experiments showed that this could in all probability have been oligomeric or polymeric sugars (Henne, Schulze, Pelger, Tretzel, and von Sengbusch: "Hollow Fiber Dialyzers and Their Pyrogenicity Testing by Limulus Amebocyte Lysates"; Pearson, Bohon, Lee, Brusser, Sagona, Dawe, Jakubowski, Morrison, and Dinarello: "Comparison of Chemical Analyses of Hollow Fiber Dialyzer Extracts"; Artif. Org. 8, 291 (1984).
Heretofore, it was only possible to reduce the portion of extractable, high-molecular weight substances by washing with water on the finished dialyzer and retaining the membrane properties by a special post-treatment (DE-OS No. 33 19 504). This process is very costly, because the treatment must be effected on each individual dialyzer.
The extractive materials, the details of which are still unknown and which show the falsely positive LAL reaction described above, are present in concentrations on the order of ng/g cellulose, and thus can be detected only by such sensitive test methods as the LAL test. The compositions of another group of extractive materials that can be detected in a concentration of less than 0.1% of the cellulose could also heretofore not as yet be clearly identified. These substances can be detected by HPLC chromatography within the molecular-weight range of 20,000 to 90,000 daltons.